1. Field of the Invention
The invention relates to a handover method in a cellular radio system comprising at least one subscriber terminal and a base station, and in which system the subscriber terminal maintains and continuously updates an active set of base stations, one or several base stations belonging to the active set having a connection or connection set-up facilities to the subscriber terminal.
The invention also relates to a cellular radio system comprising at least one subscriber terminal and a base station, in which system the subscriber terminal maintains and continuously updates an active set of base stations, one or several base stations belonging to the active set having a connection or connection set-up facilities to the subscriber terminal.
2. Description of Related Art
When a subscriber terminal moves from one cell to another in a cellular radio system, a handover is performed, and this handover is either, in a simple case, a hard handover or a more flexible soft handover. The drawback of the hard handover is that the old connection is broken before a new one is set up. The problem is more marked especially in the ping-ponging effect wherein a channel keeps changing back and forth between different channels. This can be diminished by using a handover margin. However, the best base station connection cannot be utilized in such a case due to the handover margin and the delay of the handover procedure, wherefore an unnecessarily high transmit power must be used and the disturbance power of the system thereby increases.
The prior art soft handover also employs updating margins, which are covered, however, in the reception of the subscriber terminal by signals from base stations with better audibility. Furthermore, even these unnecessary signals increase the transmit power of the base stations and interfere with the other subscriber terminals. The prior art handover is described for example in Finnish Patent Application 952,396 (Granlund, Hakkinen, Hamalainen): "Method for improving the reliability of handover and call establishment, and a cellular radio system", which is incorporated herein by reference.
As it is known, a pilot signal is used to identify a base station and to form an active set in the CDMA system. A pilot signal is a data-unmodulated spreading-coded signal, which is continuously transmitted by each base station to its coverage area. A terminal equipment can identify the base stations on the basis of the pilot signal, since the spreading codes of the pilot signals differ from one another.
Subscriber terminals continuously measure pilot signals. In order to reduce the measurement load of a terminal equipment in prior art systems, each terminal equipment maintains a measurement list of the base stations and the corresponding spreading codes of the pilot signals that are situated near the terminal equipment and that are possible candidates for handover or connection establishment. The base stations on the measurement list form a group of candidates, which may become members of the active set. It is possible to establish connections rapidly to the active set from the fixed network. Terminal equipments monitor with the highest priority the pilot signals of only those base stations that are on the measurement list.
When a terminal equipment moves, the measurement list must naturally be updated as the need arises. In the prior art systems updating is performed according to the measurement performed by the terminal equipment on the strength of the pilot signal, i.e. if a pilot transmitted by a base station is received with adequate strength, it is added to the measurement list.
A rake receiver used in particular in the CDMA system comprises several branches, each of which may be synchronized with a different signal component. The receiver can therefore receive several signals simultaneously. On the basis of the measurements of the pilot signal, the branches of the rake receiver are also caused to receive signals that arrive along different propagation paths. The rake receiver adapts to the attenuation changes over the different connections considerably faster than the active set is updated.
In a CDMA network providing many services there occur, however, situations wherein the load of the base station in the transmission direction from the terminal equipment to the base station, i.e. in the uplink transmission direction, is considerably greater than in the opposite transmission direction. An example of this is the unidirectional data transmission from the terminal equipment to the network. The prior art arrangements for updating the measurement list do not detect and therefore change the load of the base station in this transmission direction.
Even though the best signal can be selected and the power of signals which have propagated along different paths can be compiled when a rake receiver is used in the subscriber terminal, the number of the rake branches is limited by the power consumption and the manufacturing costs, and therefore the rake receiver cannot utilize, however, more than a few transmissions at a time.
Neither do the known systems provide the possibility of transferring connections to other base stations in a situation where an individual base station is overloaded.